1. Field of the Invention
The present invention relates to an improvement of an apparatus and a method for detecting an apical position applied to a root canal meter used for dental diagnosis and treatment.
2. Prior Art
Prior apparatuses used to measure root canal lengths by electrically detecting an apical position are classified into two types: a type for detecting the resistance between a measuring electrode inserted into a root canal and an oral electrode connected to an oral soft tissue (a gingiva, etc.) (refer to Japanese Patent Publication No. 62-25381 for example) and a type for detecting the impedance between the two electrodes (refer to Japanese Patent Publication No. 62-2817 for example).
The former type of the Japanese Patent Publication detects that the resistance decreases as the leading edge of the measuring electrode approaches the apical position. The latter detects that the impedance decreases as the leading edge of the measuring electrode approaches the apical position. Since the circuit between the measuring electrode and the oral electrode is assumed to be equivalent to a circuit comprising a resistor and a capacitor connected in parallel, the measurement principle of the latter type is suited for actual application. In particular, the latter type applies two signals with different frequencies between the two electrodes and detects the impedance of each signal, instead of simply detecting the impedance of the circuit, then sequentially compares the detection results to detect whether the leading edge of the measuring electrode has arrived at the apical position depending on the change in the difference between the impedance values.
The above-mentioned former type simply detects the resistance value between the two electrodes on the assumption that the inside of the root canal is dry. Therefore, if the inside of the root canal is wet, errors may occur. In actual practice, it is difficult to make measurements while the inside of the root canal is always dry. In actual clinical conditions, medical liquids and blood are present frequently in the root canal. Since the equivalent resistance in the root canal is decreased by the effect of such medical liquids, an apical position arrival mis-indication may appear even when the leading edge of the measuring electrode does not arrive actually at the apical position, or measurements may become impossible frequently. In addition, the resistance value is also affected by external factors such as the diameter of the apical foramen and the size of the measuring electrode like a file or reamer. Therefore, it is difficult to determine whether the change in the resistance value is caused by the change in the position of the file or reamer in the root canal or by other external factors, thus being apt to cause problems of improper indications.
The latter type have solved most of the above-mentioned problems. However, the type needs calibration at each measurement to eliminate the effects of the conditions in the root canal. In particular, when measuring a molar tooth with a plurality of root canals, calibration is necessary for each root canal, making operation troublesome and hindering treatment efficiency.
FIG. 4 is a graph illustrating this calibration. The abscissa of the graph indicates the position of the leading edge of the measuring electrode and the ordinate of the graph indicates the detection voltage corresponding to the impedance. The graph indicates detection values at two different frequencies f.sub.1 and f.sub.2 (f.sub.1 &lt;f.sub.2). The detection values at the higher frequency are generally larger than those at the lower frequency. Near the apical position, the rate of increase in the detection values at the higher frequency is also higher than the rate of increase in the detection values at the lower frequency. These values change up and down depending on the conditions inside the root canal.
If it is assumed that the detection values at the dental neck section are V.sub.10 and V.sub.20, that the detection values at the apical positions are V.sub.1 and V.sub.2, and that the changes in the detection values due to the change in the position of the electrode are .DELTA.V.sub.1 and .DELTA.V.sub.2, the difference between the changes (.DELTA.V.sub.2 -.DELTA.V.sub.1) is not affected by the conditions inside the root canal, thus indicating a relative change in impedance depending on the frequency. This means that the following formula is established. EQU .DELTA.V.sub.2 -.DELTA.V.sub.1 =(V.sub.2 -V.sub.20)-(V.sub.1 -V.sub.10)=(V.sub.2 -V.sub.1)-(V.sub.20 -V.sub.10)
It is therefore necessary to conduct calibration to compensate for a bias corresponding to the second term, (V.sub.20 -V.sub.10), of the above formula at each measurement by using the detection values at the dental neck section and to eliminate the effects of the conditions inside the root canal. This calibration is conducted by adjusting the offset of the detection apparatus of the latter type, for example by operating an adjustment resistor.